Homologous genetic recombination is the molecular basis for genetic exchanges upon which much of the science of genetics is based. The success of future trials in human gene therapy is also likely to depend upon our understanding of the cellular systems promoting recombination. In bacteria, the primary function of homologous genetic recombination is recombinational DNA repair. When a replication fork is halted at DNA damage, a complex system of enzymatic functions effects repair and reinitiates DNA replication. The bacterial RecA protein promotes the central steps in this process, including DNA pairing and strand exchange. Homologs of RecA protein are found in all organisms, with Rad51 protein being the best-studied eukaryotic protein in this class. The major goal of this grant is to provide a detailed understanding of the DNA strand exchange reaction promoted by RecA protein in vitro. The proposal is focused on four initiatives. The first is directed at a better understanding of the end-dependent assembly and disassembly of RecA filaments on DNA. Second the mechanism by which two DNAs are first aligned in recombination will be studied, including tests of a proposed triplex DNA pairing intermediate. Third, the activities of other DNA strand exchange proteins including other bacterial RecA proteins and the RadA protein of E. coli will be studied. An examination of these protein will help to define the range of activities that occur in this class of proteins to better relate in vitro activities to in vivo function and help test models for DNA strand exchange.